The present invention relates to a process for producing semiconductor devices comprising a silicon layer. The invention more particularly applies to the manufacture of photoelectric conversion devices, such as photovoltaic cells.
Such semiconductor devices generally have a multilayer structure, comprising at least one p-n boundary or a metal-semiconductor contact. The most frequently used basic semiconductor material is silicon. The semiconductor material layers are produced by methods involving the deposition in thin layer form on a substrate, which in particular ensures the mechanical performance of the device.
On depositing a silicon film on a non-crystalline substrate, such as silica glass by any known process a monocrystalline layer is not obtained. The layer obtained has in fact a polycrystalline or amorphous structure, as a function of the deposition conditions and particularly the temperature under which deposition takes place.
In polycrystalline layers, there are grain boundaries between the various crystals, which have the effect of impairing the electrical qualities of the semiconductor layer. If the layer is not highly n or p doped, its conductivity is generally well below that of the corresponding crystal. Moreover, the conductivity depends on the temperature according to an exponential function, which is a major disadvantage in numerous applications. One explanation which has been proposed for this phenomenon is based on the existence of potential barriers at the grain boundaries, which have to be overcome by the carriers in order to be transported through the semiconductor layer. Finally, in polycrystalline form, silicon only has a limited light absorption, so that the minimum thickness of the layer to be deposited for obtaining an adequate efficiency is several dozen microns. However, one of the main factors limiting the production of photovoltaic devices is the cost. In order to reduce cost, it is consequently desirable to use layers having a minimum thickness and of approximately only one micron. For this purpose, it has been proposed to use silicon in amorphous form, its optical absorption being significantly higher than that of crystalline silicon.
However, in general, amorphous silicon has inferior electrical conductivity characteristics to crystalline silicon, even in the case of using processes of the type described in French Patent Application No. 77 17 245, filed on June 6th 1977 and published as U.S. Pat. No. 2,394,173 and European Patent Application No. 80.401 051.0 filed on July 11th 1980 and published as Ser. No. 002,4378, which lead to improvements in these characteristics.